The present invention generally relates to a system and method for dry etching an etchable material from a substrate. The present invention more particularly relates to such a system and method wherein the substrate is transported, preferably continuously, through a plurality of chambers, wherein one of the chambers is a dry etching chamber dedicated to anisotropically etching the etchable material from the substrate, and wherein the environment within the etching chamber is isolated from immediately adjacent load and unload chambers by the maintenance of unidirectional gas flow from the adjacent chambers into the etching chamber. The present invention is particularly useful in the manufacture of large area active matrices, such as liquid crystal displays and imagers, of the type which employ a plurality of active electronic devices distributed throughout the matrix.
Amorphous silicon alloys have demonstrated a suitability for many electronic applications. Two such applications are, for example, large area active matrices of the type found in liquid crystal displays and in matrix-addressed line or two dimensional imagers. In such displays and imagers, active elements, such as PIN diodes, nonlinear switching elements, or thin film field effect transistors are associated with each display pixel or image photosensor to provide isolation between the pixels or photosensors and thus permit selective application of activating driving or charging potentials to each pixel or photosensor without the inadvertent activation of other pixels or photosensors. These amorphous silicon alloys may be made using known techniques in multiple layers successively deposited on top of the other, using plasma-assisted chemical vapor deposition (i.e., RF glow discharge) in the manner described in U.S. Pat. Nos. 4,217,374 , 4,226,898, and 4,342,044 which are hereby incorporated herein by reference.
Amorphous silicon alloys are particularly well suited for use in large area active matrices because the amorphous semiconductor alloys can be deposited as thin films over large substrate areas. High quality and virtually defect free amorphous semiconductor thin films can be formed and later selectively etched to leave discrete devices such as the aforementioned PIN diodes on the substrate. In one prior process, a top layer of metal is deposited on top of the layers of amorphous semiconductor films, and then it is patterned using conventional photoresist mask and etching techniques. Thereafter, the amorphous semiconductor films may be plasma etched using the patterned top metal layer itself as a mask, which leaves the discrete devices only under the locations where the patterned top metal remained. The devices or diodes can thereafter be connected to their respective pixel electrodes during a metallization step.
Various dry etching processes have been used or may be adapted to be used to selectively etch deposited thin films of metal, insulators, semiconductors, or combinations thereof. Well-known dry etching processes include glow discharge methods such as plasma etching, reactive ion etching (RIE) and sputter etching, and wide-area ion beam-based methods such as ion milling, chemically assisted ion milling, and reactive ion beam etching (RIBE). All of these processes use one or more process gases and all produce gaseous etch products as the material to be etched is etched from the substrate in the etching chamber. The etch products may be involatile or volatile, depending upon the material being etched and the specific process and process gases being used. Involatile etch products, and to a lesser extent volatile etch products, tend to collect or condense on surfaces within the etching chamber including the substrate. Accordingly, it is preferable to continuously remove such etch products from the etching chamber to avoid contamination of the substrate.
In glow discharge sputtering and in ion milling, the process gas or gases are typically inert (e.g., argon), and the etching is accomplished principally if not entirely by the physical abrasion and erosion of the etchable material by energetic gas molecules or ions. In plasma etching, RIE, chemically assisted ion milling, and RIBE, at least one of the process gases used is reactive. Typical reactive gases include CF.sub.4, Cl.sub.2, and XeF.sub.2. Accordingly, these later four processes are either partially or completely chemically assisted or driven. When using any of these reactive gas-based dry etching methods, the length of time activated ion species are in contact with the material to be selectively etched must be carefully controlled, as must the relative concentration of such species, to obtain the desired etching results. There are advantages and drawbacks associated with each of various methods which may be used to etch different types of thin film materials, which render certain of these methods more suitable than others for particular types of materials or structures to be selectively etched in order to form various microelectronic structures. A discussion of such advantages and drawbacks, and a further description of the aforementioned dry etching methods is provided in J. W. Coburn, "Pattern Transfer", Solid State Technology, (April, 1986), pp. 117-122.
Dry anisotropic etching processes are becoming increasingly popular because of the need for more sharply defined and densely packed features and devices in modern microelectronic structures of all types. One such anisotropic etching process which is now in wide use is reactive ion etching. The assignee of the present invention has found that reactive ion etching is particularly suitable for selectively etching deposited thin film structures including one or more layers of amorphous silicon alloy semiconductor material. For the purpose of the present disclosure RIE may be considered exemplary of the various dry etching methods.
In such a reactive ion etching process, a substrate having the deposited thin films thereon is loaded into an etching chamber. Within the etching chamber is a main electrode to which radio frequency energy is applied and a counter electrode spaced from the main electrode. The substrate is placed immediately adjacent the main electrode between the main electrode and the counter electrode. An etching process gas suitable for etching the metals, insulators, or semiconductor materials to be etched is then fed into the chamber. The radio frequency energy causes a plasma to be formed from the etching process gas which disassociates into a number of active species including usually both negative and positive ions. For example, where the etching gas is comprised of carbon tetrafluoride (CF.sub.4), negative fluorine ions and positive C, CF, CF.sub.2, and CF.sub.3 ions are formed in the plasma. Because the negative fluorine ions are lighter and more mobile than the positive ions, the main electrode develops a negative charge. This causes the positive ions to be accelerated towards the main electrode and bombard the substrate. As a result, the impinging positive ions etch the exposed areas of the thin films deposited on the substrate. Reactive ion etching is desirable because it is a dry and anisotropic process wherein the remaining thin film portions are well defined with minimum under-cutting.
In U.S. Pat. No. 4,680,085 issued July 14, 1987 in the names of M. Vijan, J. McGill, and P. Day, and entitled Method Of Forming Thin Film Semiconductor Devices, which is hereby incorporated herein by reference, there are disclosed preferred methods for reactive ion etching thin film metal and semiconductor layers, such as a amorphous semiconductor current-control devices like PIN diodes, NIN or N-pi-N nonlinear switches and the like, to form mesa-like semiconductor structures having substantially vertical sidewalls free of overhangs or under-cutting and voids. One such preferred method for the reactive ion etching of multiple layers of amorphous silicon alloy of different conductivity types to form current control devices such as a-Si:H PIN diodes involves the use of a pure CF.sub.4 process gas substantially free of all O.sub.2 in a reaction chamber which has been purged with an inert gas plasma to eliminate any residual oxygen or water before beginning the reactive ion etching step. Specific process parameters for carrying out the preferred reactive ion etching method to etch such multiple layers are disclosed in the aforementioned application, and may be profitably employed to best utilize the apparatus of the present invention and to practice the methods of the present invention on such amorphous silicon alloy layers.
While dry etching processes such as reactive ion etching have proven desirable, they have heretofore suffered some drawbacks when incorporated into commercial high volume processes. For example, two dimensional etching uniformity across the substrate is essential and has been difficult to obtain on large area substrates, e.g., 4 inch by 6 inches, 7 by 11 inch, or 12 by 12 inch substrates. This problem will be magnified when attempting to etch even larger substrates such as those which will likely be used in the not-too-distant future to make flat panel liquid crystal television sets having a diagonal dimension of 19 inches, 25 inches or more. In addition, throughput has also been a problem because, in the past, separate processing of each substrate using batch processing equipment with load-lock chambers has been required. The opening and closing of the load-locks, the pumping down of the chambers, and re-establishment of the etching plasma with every new substrate or batch of substrates placed in the etching chamber requires extra processing time, which is a very important consideration in commerical applications.
In short, the dry etching art, including the reactive ion etching art, has not kept pace with the art of depositing amorphous semiconductor thin films wherein such semiconductor thin films can now be deposited by continuous processes. A continuous dry etching process would be advantageous because it would not require two dimensional uniformity, since spatial uniformity in the direction of substrate movement would be essentially inherent or automatic under a continuously maintained and stable etching process. Thus only spatial uniformity in the direction transverse to substrate movement need be considered. Thus, for example, spatial uniformity across the rectangular substrate of a 19 inch or 25 inch diagonal liquid crystal television display would only have to be maintained across the width of the substrate. Continuous dry etch processing such as reactive ion etching would also be advantageous since it would lend itself to high volume mass production of large area substrates and, unlike present batch dry etching systems and methods (such as RIE batch systems and methods), would be better able to keep up with processing throughput demands imposed by the continuous deposition systems which deposit the high quality amorphous semiconductor thin films and metals to be etched.
The topic of providing a continuous dry etch system or method for etching etchable material such as semiconductor thin films on substrates has apparently not been addressed by those in the art. One possible reason for this is that any such system or method used in conjunction with a semiconductor deposition system must strictly avoid contamination of the semiconductor deposition system by the etching system.
The field of thin film deposition systems and methods is quite distinct and different from the field of dry etching systems, even though both employ plasma systems and vacuum technology, since one field is devoted to depositing materials, while the other field is devoted to removing materials, and the processes, chemistry, and process parameters differ tremendously. In the thin film deposition field, the present inventor and others affiliated with the assignee of the present invention have developed systems, methods and devices for depositing amorphous silicon alloys in high volume continuous processing systems for photovoltaic uses. Systems of this kind are disclosed for example in U.S. Pat. No. 4,400,409 issued Aug. 23, 1983, for A Method Of Making P-doped Silicon Films, U.S. Pat. No. 4,542,711 issued Sept. 24, 1985, for Continuous Systems For Depositing Amorphous Semiconductor Material, and U.S. Pat. No. 4,410,558 issued Oct. 18, 1983, for Continuous Amorphous Solar Cell Production System. As disclosed in these three patents, which are hereby incorporated herein by reference, a substrate is continuously advanced through a succession of deposition chambers with each chamber being dedicated to the deposition of a specific type of material. In making a solar cell of PIN configuration, for example, a first chamber is dedicated for depositing a p-type amorphous silicon alloy, a second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and a third chamber is dedicated for depositing an n-type amorphous silicon alloy. In order to make such a solar cell of maximum efficiency, each deposited alloy, and especially the intrinsic alloy, must be of high purity. As a result, it is necessary in such systems to isolate the deposition environment in the intrinsic deposition chamber from the doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber. In the systems disclosed in the aforementioned patents, such isolation between the chambers is accomplished with gas gates which pass an inert gas over the substrate as it passes from one chamber to the next.
An improvement to the systems disclosed in the above-mentioned patents is disclosed in U.S. Pat. No. 4,438,723 issued Mar. 27, 1984, for Multiple Chamber Deposition and Isolation System and Method, which is hereby incorporated herein by reference. In the continuous processing system disclosed in this patent, the deposition environment within the intrinsic amorphous silicon alloy deposition chamber is isolated from the deposition environments within the doped amorphous silicon alloy deposition chambers adjacent thereto by the maintenance of unidirectional gas flow from the intrinsic deposition chamber into the chambers adjacent thereto. As disclosed, it is preferred that the unidirectional gas flow be maintained through the gas gates through which the substrate is transported from one chamber to the next.
The references relating to the production of photovoltaic devices such as solar cells do not disclose or suggest how to adapt the systems or methods taught therein to dry etching of etchable materials such as layers of amorphous semiconductor alloy thin films from substrates.